This invention relates to apparatus for providing protection against thermal overloads in electric discharge lamp fixtures or the like, and more particularly to a thermal overload protection device especially adapted for use with a dual input voltage transformer/ballast apparatus for operation of electric lamps.
A provision set out in the National Electric Code 1987 version requires that certain fixtures must be thermally protected, e.g. indoor, recessed HID lamp fixtures. If this type of fixture is accidentally or otherwise covered by insulating material, or insulation material is placed too close to the fixture, then there is a substantial likelihood that the heat developed in the fixture will ignite the flammable insulation material and thereby cause a fire, with the attendant danger of a loss of property and/or life. Hence, the above addition to the National Electric Code of a requirement for a thermal overload protection device for a recessed HID lamp fixture.
A thermal protector is available that is designed to operate only with an AC supply voltage of 120 volts (60 Hz). However, the most popular ballast devices for use with HID lamp fixtures are rated for dual operation with input voltages of either 120 V or 277 V.
In order to provide thermal protection for a dual input voltage HID lamp ballast, it has been suggested to modify the transformer/ballast in the manner indicated in FIG. 1 of this application. A HID lamp ballast 1 is indicated diagrammatically and contains a transformer 2 having a primary winding 3 with the connections indicated by the dashed line. That is, a primary winding consisting of all of the winding turns between the terminals Y and Z. In order to modify this ballast device for use with dual input voltages of 277 V and 120 V and provide it with thermal overload protection, it has been proposed to break the connection shown in dashed lines and to connect the terminal 4 of the primary winding to an input terminal 5. A thermal switch consisting of a normally closed contact arm 6 (e.g. a bimetallic element), operative in response to heat applied thereto by a heating resistor 7, is connected to the terminal 5. The terminal X of the primary winding is now connected to the output contact of the normally closed switch. The heating resistor 7 is connected across the part of the winding between the terminals X and Z.
If the ballast device is to be used with a 277V input voltage, then the terminals 8 and 9 (connected to terminals Y and Z, respectively, of the transformer winding) are connected to the terminals of the 277 V supply voltage. In case of operation with a 120 V source, the terminals 5 and 9 are connected to the 120 V supply lines. In the event of a thermal overload, the normally closed contact arm 6 responds by opening the line to terminal X of the transformer winding.
The proposed circuit of FIG. 1 is subject to several limitations that make it relatively unattractive and impractical as a solution to the thermal overload problem. First of all, it requires a significant modification of the ballast device as a result of breaking the normal connection of the winding at point X thereof. In addition, with some ballast auto-transformers having taps and lamp loads above the voltage at terminal X, the opening of the normally closed switch arm 6 will not cut off the power to these loads. Furthermore, the job of a field installation technician is complicated by an additional lead from the transformer winding to terminal 5 etc.